I was thinking if my 800 W PSU would be too low for this setup since 5850 is getting only 400MHz to GPU core and 900 MHz to memories and I can't change them in CGminer even I try to lower the memory and increase the GPU MHz.

It's saying voltage 1.0V for 5850 and 1.175 for 6970.

5850 is giving 150MHash/s and 6970 is giving 400MHash/s (was giving 430Mhash/s on its own)What could I try out?

When I'm running this 5850 on it's own I can only achieve : GPU 820 MEM 300 VDCC 1.088. If i'll lower mem / vdcc it will crash. If I'll raise the GPU the drivers will crash.At the moment it's running with setup mentioned before and fan @ 3375RPM & 72°C.

Do I'm planning to put 6970 back to see if it's back to same, but according to those numbers this card is quite a failure (5850) at least what I have stalked what others have?

Should I just try to cancel my 60€ trade for this card or is anyone else interested to buy this off from me with that price?

Try using Sapphire trixx to adjust the 5850. I have one computer running a 5870, 5970 (2 cores), & 6950. The 6950 starts out like your 5850 with just cgminer, I installed trixx since I use it on the other computers and I was able to adjust it up. Also just had an issue with the 5870 where cgminer wouldn't show the changes I made even though they were being applied (as reported by trixx)

Just an FYI, my rig listed above (4 GPU's) is running perfectly on a 1000W power supply. Your 800 should be more than enough. I have measured it at the wall before at less than 800W, but that was with a 5850 instead of the 6950, but the difference should be negligible. I switched my incoming power to 240V and my kill a watt doesn't support that.

Are you certain that you have installed cgminer correctly? I tried to get it running on crunchbang linux, and I did get the cards running. But they were hashing at half the normal rate. After that, I went to a version of ubuntu 11.04, and followed kano's instructions to the letter for installation of cgminer, and things are working perfect now. These are the instructions I'm talking about:

Before assuming you have a bad 5950, I would grab a version ubuntu 11.04 (I used Peppermint Two), and follow these instructions. The overall point is that just because you have cgminer installed and it fires up your cards, this doesn't mean that you have it installed correctly.

I thought it was calculated the other way around? It should provide 750 watts of DC power. Due to inefficiencies, it will draw more AC power at the wall and expel this inefficiency as heat. 80% efficient 750w psu at full load should output 750w of DC power and 150w of heat (900w AC power)

on topic though, to respond to OP, yes 800w PSU should be well above and beyond what you need for those two cards. If it is a single rail PSU, you are golden and have huge headroom, enough to porbbaly hook up 2 more cards on top of that. If it is multi rail though, you might be screwed, as you will have power "locked" to certain rails and unavailable to use to your cards. I have a 550w PSU thats multirail, and it refuses to power any more than 2 5830's severely undervolted and underclocked without turning off. I also have an old 20-pin ATX 600watt power supply that will happily power 3 cards at normal speeds because it isnt multi-rail

Multi-rail is bad, it provides too much hastle and problems with balancing load. Single rail PSU's you just plug in and go, no problem.